UNIVERSITY   OF   CALIFORNIA 

COLLEGE    OF    AGRICULTURE 
AGRICULTURAL    EXPERIMENT   STATION 

CIRCULAR  No.  216 
November,  1919 

WINTER  INJURY 

OR 

DIE-BACK  OF  THE  WALNUT 

By  L.  D.  BATCHELOE  and  H.  S.  BEED 


This  circular  contains  an  account  of  studies  made  throuh  sev- 
eral seasons  on  the  so-called  "Die-back"  or  "Winter  Injury"  of  the 
Persian  walnut  (Juglans  regia).  In  certain  districts  where  walnuts 
are  commercially  grown,  the  trees  have  suffered  greatly  from  this 
trouble,  and  in  some  cases  parts  of  orchards  have  died  from  this  cause. 

Unfavorable  soil  or  climatic  conditions  which  would  show  little  or 
no  effect  upon  many  species  of  orchard  trees,  may  seriously  injure,  if 
not  actually  kill,  a  walnut  tree.  The  effect  of  such  unfavorable 
conditions  is  often  first  clearly  manifested  in  the  early  spring.  At 
that  time  the  tops  of  healthy  trees  make  rapid  growth,  whereas  the 
tops  of  injured  trees  show  no  growth  whatever.  Frequently  unfavor- 
able conditions  not  only  cause  the  death  of  the  uppermost  twigs  but 
also  of  some  of  the  older  branches. 

"Winter  Injury"  is  a  term  frequently  applied  to  several  con- 
ditions, the  effects  of  which  are  particularly  evident  in  the  early 
spring.  Usually  "Winter  Injury"  is  most  clearly  manifested  by  the 
death  of  the  young  twigs  and  small  limbs  and  hence  is  frequently 
known  as  ' '  Die-back. ' '  These  terms  will  be  considered  as  synonymous 
in  the  following  discussion. 

Winter  injury  may  be  considered  as  the  final  effect  of  one  or 
more  conditions  which  are  adverse  to  the  normal  growth  of  walnut 
trees  and  which  eventually  cause  them  to  die  back  in  the  tops.  The 
following  are  the  most  common  causes  of  winter  injury:  (1)  Early 
autumn  frosts,  (2)  winter  drought,  (3)  high  water-table,  and  (4) 
alkali  soil. 


1  The  writers  are  pleased  to  acknowledge  the  assistance  given  by  Mr.  D.  C. 
Wylie  of  the  Field  Department  of  the  California  Walnut  Growers '  Association  and 
by  Mr.  E.  E.  Thomas  of  the  Citrus  Experiment  Station. 


1.  Early  autumn  frosts. — Young  trees  are  more  likely  to  suffer 
from  early  frosts  than  are  mature  bearing  trees  because  the  latter  go 
into  a  dormant  condition  earlier  in  the  season.  Walnut  foliage  which 
is  still  green  and  shows  no  sign  of  its  normal  autumn  color,  is  appar- 
ently as  subject  to  frost  injury  as  are  tender  vegetables.  Such 
injured  foliage  drops  prematurely,  and  hence  the  green,  immature 
twigs  are  fully  exposed  to  the  sun's  heat  during  bright  days  in  fall 
and  winter.  The  moisture  loss,  following  the  dropping  of  the  leaves, 
is  much  more  rapid  from  the  succulent  twigs  than  from  the  more 
mature  growth. 

In  many  cases  the  twigs  themselves  are  not  actually  injured  by 
the  frost,  but  are  killed  by  sunburning  following  the  loss  of  the  leaves. 
This  is  substantiated  by  the  fact  that  such  twigs  usually  begin  to 
show  injury  from  sunburning  about  the  middle  of  January.  By 
early  spring,  more  or  less  of  the  new  growth  is  dead.  In  occasional 
instances,  however,  a  portion  of  the  north  side  of  twigs  thus  injured 
may  remain  healthy  and  produce  normal  growth  in  the  following 
spring.  The  death  of  the  immature  twigs  is  due  not  so  much  to  the 
initial  low  temperature  which  killed  the  leaves  as  to  the  subsequent 
sunburning  of  the  exposed  bark.  Vertical  young  shoots  frequently 
show  discolor ations  due  to  sunburn  on  the  south  side  over  their  entire 
length,  while  horizontal  or  oblique  branches  usually  show  this  sun- 
burned area  more  strikingly  on  their  upper  sides.  Such  a  sunburned 
appearance  of  the  young  limbs  in  the  spring  is  a  prevailing  character- 
istic of  trees  which  have  been  only  mildly  frosted  the  previous  fall 
before  the  dormant  period  set  in.  In  as  much  as  the  root  systems 
are  not  injured  by  this  condition,  such  frosted  and  sunburned  trees, 
as  a  general  rule,  produce  a  rank,  succulent  twig  growth  the  summer 
following  the  initial  injury.  Such  growth  in  turn  is  less  likely  to 
mature  early  and  more  likely  to  be  injured  by  autumn  frosts  than 
the  normal  tree.  When  the  cycle  of  events  just  described  becomes 
established,  it  thus  tends  to  perpetuate  itself. 

If  the  trees  are  kept  thoroughly  covered  with  whitewash  during 
the  winter  months,  the  injury  resulting  from  sunburn  may  be  some- 
what reduced.  Such  a  treatment  tends  to  maintain  a  lower  tempera- 
ture within  the  twigs.  Too  much  reliance  should  not  be  placed  upon 
this  treatment,  for  it  has  been  observed  that  at  best  it  affords  on\y 
partial  protection.  As  a  general  rule  it  has  been  observed  that  frost 
injury  to  the  foliage  of  immature  twigs,  is  practically  certain  to  be 
followed  by  die-back  to  some  extent,  in  spite  of  any  remedy  known 
to  the  writers  at  the  present  time. 


The  coincidence  of  frosted  foliage  and  subsequent  die-back  in  a 
five-year-old  grove  is  shown  by  Chart  I.  The  new  wood  on  these 
trees  was  nearly  mature  when  the  first  frost  occurred  in  November, 
1918.  The  observations  on  die-back  were  made  in  May,  1919.  The 
majority  of  the  trees  that  had  been  frosted  showed  die-back  injury; 


Fig.  1. — A  walnut  tree  injured  by  frost  the  preceding  autumn.     The  orchard 
lias  had  excellent  care  but  is  located  on  low  ground. 


none  were  frosted  without  subsequent  die-back.  Two  trees  which  were 
not  affected  by  the  first  frost  still  showed  a  slight  injury  from  die- 
back.  In  a  few  cases,  however,  it  was  very  apparent  from  the  black- 
ened condition  of  the  twigs,  especially  the  tips,  that  the  wood  had 
been  actually  killed  by  the  frost  at  the  time  of  the  foliage  injury. 
In  such  instances  as  the  last  it  is  clear  that  remedial  measures  will 
be  of  no  avail. 

Figure    1   shows   a   walnut   tree    grown    under    apparently   good 
environmental  conditions  in  all  respects,  except  that  the  orchard  site 


is  on  low  ground  and  the  trees  are  injured  by  frost  before  the  wood 
matures  in  the  autumn.  The  healthy  condition  of  the  roots  is  mani- 
fested by  the  abundant  growth  of  suckers  from  the  root-stock. 

Chart  I. — The  Eelation  Between  Autumn  Frost  Injury  and  Die-Back  in  a 
Five-Year-Old  Walnut  Grove 


10 

9 

8 

7 

6 

5 

4 

3 

2 

1 

X 

X 

X 

X 

X 

0 

0 

X 

- 

0 

xxxxxxxxxo 

Die-back  trees,  -;  normal  trees,  o;  trees  showing  both  frost  injury  and  die- 
back,  x. 

Injury  from  frost  may  be  prevented  if  it  is  possible  to  mature  the 
young  walnut  trees  early  in  the  autumn  so  that  the  leaves  will  turn 
yellow  and  normally  fall  from  the  tree  before  the  first  early  frosts 
which  usually  occur  from  the  1st  to  the  15th  of  November,  in  many 
of  the  lowlands  in  southern  California.  Early  maturity  can  be 
promoted  by  withholding  irrigation  water  in  the  late  summer  and  early 
fall.  Trees  which  are  not  irrigated  after  August  15th  may  be  expected 
to  mature  earlier  than  trees  watered  in  late  August  and  September. 
Whenever  the  soil  moisture  is  abundant  in  the  autumn,  due  either 
to  heavy  summer  irrigation  or  to  a  high  water-table,  the  trees  will 
continue  their  growth  so  late  as  to  be  injured  by  autumn  frosts.  A 
high  water-table  and  a  frosty  orchard  site  make  winter  injury  a  prac- 
tical certainty.  It  is  doubtful  if  walnuts  can  ever  be  successfully 
grown  on  such  lands.  Figure  2  shows  a  walnut  tree  growing  in  a  rich, 
deep,  sub-irrigated  soil  in  a  frosty  location. 

2.  Winter  drought. — We  have  very  convincing  evidence  to  show 
that  trees  which  enter  the  dormant  period  in  the  fall  in  a  perfectly 
normal  and  healthy  condition  may  suffer  from  die-back  due  primarily 


to  a  lack  of  sufficient  soil  moisture  during  the  winter  months  (fig.  3). 
During  the  winter,  trees  give  off  moisture  through  the  limbs  and 
twigs.  If  for  a  prolonged  period  there  is  not  enough  soil  moisture 
available  to  the  roots,  the  trees  are  unable  to  obtain  sufficient  water 
to  offset  the  loss  by  evaporation  from  the  branches.     In  that  case 


Fig.  2. — A  walnut  tree  growing  in  a  rich,  deep,  sub-irrigated  soil  in  a  frosty- 
location.     The  new  growth  continued  to  grow  very  late  and  was  killed  by  frost. 


young  branches,  the  thin  bark  of  which  permits  rapid  loss  of  water 
from  the  wood,  may  die  as  a  result  of  desiccation.  This  injury  is  first 
evident  when  such  branches  fail  to  produce  new  growth  the  following 
spring. 

It  is  usually  not  difficult  to  distinguish  between  the  injuries  caused 
by  autumn  frosts  and  by  winter  drought.  Frost  injury  is  usually 
confined  to  one  or  two-year-old  wood,  but  winter  drought  may  kill 
back  limbs  eight  years  old.     Winter  sunburning  is  more  common  on 


shoots  killed  by  autumn  frosts  than  on  those  killed  by  winter  drought, 
though  the  sunburned  areas  may  not  be  clearly  defined  until  January. 
The  principal  contributing  factors  which  bring  about  such  a 
condition  are  dry  winters  and  the  lack  of  winter  irrigation.  Under 
such  conditions  the  soil  may  have  little  or  no  moisture  available  for 


Fig.  3. — A  tree  killed  back  two  consecutive  years  by  winter  drought. 

the  tree  roots  below  the  surface  foot  and  even  this  zone  does  not 
contain  an  optimum  amount  of  moisture  until  mid-season  of  the  winter 
rainy  period. 

The  winter  of  1917-18  was  followed  by  a  great  deal  of  die-back 
injury  to  bearing  walnut  trees.  An  examination  of  walnut  orchards 
during  this  winter  showed  that  most  of  the  soils  were  very  dry.  In 
several  cases  in  orchards  which  subsequently  showed  die-back  injury, 
the  moisture  in  the  first  four  feet  of  soil  was  below  the  wilting  point 


until  late  winter.  Thus  the  roots  were  unable  to  acquire  sufficient 
moisture  to  replenish  the  loss  from  the  trees  and  consequently  the  tips 
were  killed  by  excessive  desiccation. 

The  result  of  this  winter  drought  can  be  judged  by  figure  4,  which 
is  a  general  view  in  a  grove  showing  the  effect  of  die-back  in  May,  1918. 
During  the  fall  of  1918  the  dead  wood  in  these  trees  was  removed  and 
the  grove  was  thoroughly  irrigated  during  the  latter  part  of  November. 
Figures  5  and  6  show  the  same  trees  in  November,  1918,  and  July, 
1919,  respectively.  The  trees  are  fast  recovering  from  the  winter 
injury  and  were  killed  back  only  slightly  during  the  winter  of  1918-19 


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Fig.  4. — A  general  view  in  a  grove  which  suffered  from  drought  during  the 
winter  of  1917-18.     Photo  taken  in  May,  1918. 

following  the  irrigation.  The  trees  which  were  the  most  severely 
injured  the  previous  year  and  thus  produced  sucker  growth  from  six 
to  ten  feet  long,  died  back  at  the  tips  during  1918-19.  Such  excessive 
growth  was  very  immature  as  late  as  December,  1918,  while  the 
succulent  foliage  on  the  tips  had  been  frosted  early  in  November. 

We  have  found  that  winter-injured  trees  are  more  susceptible  to 
die-back  the  year  following  the  initial  injury,  than  are  normal  trees. 
The  increased  susceptibility  of  injured  trees  is  inevitable  because  of 
the  succulent  nature  of  much  of  the  young  growth  produced.  In  the 
summer  following  the  first  "Die-back,"  the  amount  of  living  wood 
is  small  in  comparison  with  the  root  system,  and  is  forced  into  very 
active  growth.  The  type  of  growth  produced  is  therefore  similar  to 
that  following  a  severe  pruning  of  the  tree. 

The  majority  of  the  trees  in  the  Steinburg  grove  (Hemet,  Calif.) 
leafed  out  nearly  to  the  tips  of  the  twigs  in  the  spring  of  1919,  follow 


8 

ing  the  winter  irrigation.  Figures  7,  8,  and  9  show  a  typical  badly 
injured  tree  in  this  grove,  and  the  stages  in  its  subsequent  recovery 
as  a  result  of  the  winter  irrigation.    The  pictures  were  taken  May  23, 


Fig.  5 


Fig.  6 

Figs.  5  and  6. — Pictures  of  the  same  orchard  shown  in  fig.  4. 
taken  in  November,  1918,  and  July,  1919,  respectively. 


General  view 


1918,  November  20,  1918,  and  August  13,  1919,  respectively.  The 
dead  wood  seen  in  figures  7  and  8  was  removed  from  the  trees  after 
photographing  it  in  November,  1918.  From  the  results  it  will  be  seen 
that  the  wood  which  went  into  the  winter  in  healthy  condition  came 


through  without  injury  while  in  the  absence  of  winter  irrigation  it 
would  undoubtedly  have  gone  back  to  the  condition  shown  in  figure  7. 
Winter  injury  as  illustrated  above,  which  is  apparently  traceable 
to  lack  of  sufficient  soil  mosture,  can  often  be  seen  in  groves  which  are 
not  irrigated  after  the  walnut  harvest  season.  Frequently  such  groves 
show  a  direct  relationship  between  the  distribution  of  the  injured 
trees  and  the  available  soil  moisture  during  the  winter  season.  Cases 
of  this  class  may  be  clearly  exemplified  by  the  0.  W.  Morrow  grove 


Fig.  7. — A  typical  badly  injured  tree  in  the  Steinburg  grove.     Picture  taken 
May  23,  1918, 


of  Hemet.  This  grove  received  no  irrigation  after  October,  1918.  A 
small  quantity  of  water  was  used  (37.5  miners'  inches  per  ten  acres 
for  24  hours)  and  was  applied  in  furrows  uniformly  spaced  three  feet 
apart.  Since  the  irrigation  furrows  were  600  feet  long  it  is  evident 
that  the  lower  part  of  the  orchard  did  not  receive  sufficient  water 
during  the  twenty-four  hours'  run,  though  the  upper  portion  was 
fairly  well  watered.  This  irregular  distribution  of  water  was  evident 
as  late  as  May  15,  1919  (before  the  next  irrigation  season  began).  It 
should  be  noted  that  the  moisture  in  the  soil  at  such  a  time  consists 
of  that  left  over  from  preceding  irrigations   plus   that   which   has 


10 


accumulated  from  the  winter  rains.  Our  determinations  showed  that 
there  was  a  marked  difference  in  the  amounts  of  moisture  in  the  upper 
and  lower  parts  of  this  grove.  In  the  upper  part  of  the  grove  the  soil 
was  moist  to  a  depth  of  five  to  six  feet,  while  in  the  lower  part  of  the 
grove  the  moisture  did  not  extend  much  below  three  feet  from  the 
surface.2 

The  relation  between  soil  moisture  and  winter  injury  can  be  seen 
at  a  glance  in  the  orchard  and  is  illustrated  by  Chart  1.     For  con- 


Fig.   8. — The   same  tree  shown  in  fig.   7.     Picture  taken  November   20, 
The  dead  wood  was  removed  from  the  tree  after  photographing. 


1918. 


venience  in  observing  the  winter-injury  in  this  orchard,  we  have 
divided  the  trees  into  three  classes :  first,  trees  not  winter-killed ;  sec- 
ond, trees  slightly  winter  killed,  and  third,  trees  badly  winter-killed. 
Reference  to  Chart  2  shows  that  the  winter  injury  was  greatest  in  the 


2  By  the  use  of  a  soil  tube  the  penetration  of  the  winter  rains  or  of  irrigation 
water  can  easily  be  determined.  The  subsoils  of  this  district  in  the  Hemet 
valley  are  free  from  any  ground  water  for  many  feet  from  the  surface  and  are 
characterized  by  their  ability  to  be  reduced  by  native  vegetation  or  orchard  trees, 
to  a  characteristic  dryness  easily  recognized.  This  dryness  approaches  close  to 
the  hygroscopic  coefficient  throughout  the  subsoils  at  the  end  of  each  growing 
season.  In  strata  below  the  reach  of  irrigation  water  or  rainfall,  the  subsoil  may 
remain  permanently  in  this  dry  condition. 


11 

lower  section  of  the  grove,  less  in  the  middle  section,  and  least  in  the 
upper  section.  The  two  tree-rows  in  this  orchard  below  the  lower 
section  show  less  injury  than  the  section  just  above  them,  due  appar- 
ently to  the  fact  that  no  waste  water  is  allowed  to  run  off  this  orchard, 
but  is  held  by  a  dike  and  thus  the  lower  two  rows  get  an  extra  amount 
of  water.3 


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Fig.  9. — Same  tree  as  shown  in  figs.  7  and  8.  Picture  taken  August  13,  1919. 
The  picture  shows  that  the  tree  made  an  almost  complete  recovery  following  winter 
irrigation. 


The  actual  number  of  trees  injured  in  each  section  was  as  follows : 

No  slight  Badly 

injury  injury  injured 

Upper   section   48  4  0 

Middle  section 12  26  14 

Lower   section*    1  16  34 

*  One  tree  is  missing  in  this  section. 

This  distribution  and  the  fact  that  it  cannot  be  due  to  chance,  is 
clearly  set  forth  by  the  diagram.  A  soil  survey  of  this  orchard  shows 
no  appreciable  difference  in  the  surface  soil  or  subsoil  types  within 

3  This  orchard  has  been  normally  irrigated  at  the  rate  of  only  1.8  acre  inches 
per  irrigation  per  month  from  May  to   October. 


12 

seven  feet  of  the  surface.  This  and  other  examples  which  have  been 
studied,  point  clearly  to  a  lack  of  soil  moisture  as  the  important  factor 
in  causing  winter  injury  of  the  walnut  trees  in  this  and  many  other 
orchards. 

Chart  2. — Distribution  of  Winter-Killed  Trees,  O.  W.  Morrow  Grove, 
Hemet,  May  15,  1919 


IRRIGATION 

FLUME 

Upper   section 

0 

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Middle   section 

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* 

Lower   section 

X 

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* 

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* 

* 

* 

0 

* 

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X 

* 

* 

X 

* 

X 

X 

* 

* 

* 

* 

* 

X 

* 

* 

* 

* 

* 

X 

X 

* 

* 

* 

* 

* 

* 

* 

* 

* 

0 

X 

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* 

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* 

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* 

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0 

HIGHWAY 

0,  trees  not  winter-killed;  x,  trees  slightly  winter-killed;   *,  trees  badly  winter- 
killed. 


In  view  of  results  such  as  those  presented  in  the  foregoing  pages 
it  seems  evident  that  it  is  not  difficult  to  prevent  this  type  of  die-back, 
or  winter  injury,  due  to  winter  drought.  The  application  of  sufficient 
irrigation  water  to  reach  the  tree  roots  soon  after  the  harvest  season 
will  tend  to  prevent  such  injuries.    The  grower  should  use  a  soil  tube, 


13 

or  a  soil  auger,  in  the  autumn  to  determine  whether  the  soil  to  a  depth 
of  five  feet  contains  enough  moisture  for  the  welfare  of  the  tree  during 
the  winter.  The  regular  practice  of  fall  irrigation  in  walnut  groves 
may  be  considered  as  a  profitable  form  of  insurance  since  it  is  im- 
possible to  predict  the  occurrence  or  the  volume  of  winter  rains. 
We  know  that  frequently  the  rains  are  too  late  and  insufficient  for  the 
welfare  of  walnut  groves.    It  goes  without  saying  that  trees  on  a  light 


Fig.  10. — Shows  winter  drought  injury  in  a  Tustin  grove  photographed  July  30, 
1919.  The  soil  moisture  in  this  grove  was  reduced  in  the  preceding  fall  by  the 
inter-crops.     The  trees  were  badly  injured  before  the  winter  rains  occurred. 


sandy  type  of  soil  are  in  greater  danger  of  injury  from  winter  drought 
than  are  trees  on  a  heavier  type  of  soil.  In  addition  to  soil  types 
there  are  other  factors  involved  in  the  production  of  winter  injury, 
such  as,  the  character  of  the  rainfall,  the  rate  and  amount  of  evapora- 
tion, temperature,  etc. 

The  growth  of  a  winter  inter-crop  such  as  barley,  during  a  normal 
season  may  bring  about  the  same  adverse  soil  moisture  condition  that 
the  dry  winter  does,  unless  great  care  is  taken  to  apply  enough  water 
for  the  needs  of  both  trees  and  inter-crop.  For  example,  if  the  barley 
is  considered,  the  soil  may  become  excessively  dry  about  the  time 


14 

that  the  hay  is  maturing,  which  is  the  time  the  walnuts  should  be 
starting  into  active  growth. 

Unless  fall  irrigation  is  practiced  the  growth  of  summer  inter-crops 
between  the  walnut  trees  may  be  conducive  to  winter  injury  of  the 
trees.  This  is  due  to  the  fact  that  the  moisture  is  reduced  to  such  an 
extent  at  the  end  of  the  growing  season  that  the  trees  are  injured 
before  the  winter  rains  occur.  Such  a  case  is  well  shown  by  an  orchard 
of  three-year-old  walnuts  in  Tustin  which  was  interplanted  to  peppers 
(fig.  10).  The  peppers  and  trees  were  last  irrigated  August  28,  1918. 
There  were  no  early  fall  frosts  to  complicate  the  case,  as  shown  by  the 
fact  that  peppers  were  harvested  as  late  as  November  15  in  this 
grove.  In  the  spring  of  1919  the  majority  of  the  trees  showed  serious 
winter  injury  throughout  the  tops.  Without  question,  the  soil  moist- 
ure during  practically  the  entire  dormant  period  of  the  trees  was 
considerably  below  the  wilting  point,  except  in  the  surface  foot  of 
soil.  The  winter  rains  of  6.97  inches  which  fell  in  small  intermittent 
amounts,  penetrated  the  soil  in  nearby  bean  fields  to  the  extent  of 
only  twelve  to  eighteen  inches  at  the  end  of  the  rainy  season. 

The  same  results  are  often  found  where  beans  have  been  grown 
as  an  intercrop.  Water  is  withheld  from  such  groves  during  the  latter 
part  of  the  summer  in  order  to  ripen  the  beans.  After  the  beans  are 
harvested  the  soil  of  the  grove  is  usually  very  dry.  This  condition 
may  be  of  benefit  to  the  walnut  trees  because  it  insures  a  state  of 
maturity  which  will  bring  them  through  the  winter  in  good  condition. 
In  such  cases,  however,  winter  irrigation  is  usually  necessary  and 
should  be  applied  soon  after  the  leaves  fall  from  the  walnut  trees. 

3.  High  water-table. — A  high  water-table  may  be  a  contributing 
factor  to  the  die-back  of  walnuts.  When  such  a  condition  exists  it  is 
usually  impossible  to  bring  about  the  early  maturity  of  the  walnut 
trees  by  withholding  the  late  summer  and  fall  irrigations. 

In  the  presence  of  a  high,  more  or  less  stationary  water  table,  the 
twigs  and  foliage,  especially  on  young  trees,  usually  remain  green  and 
succulent  until  December,  unless  killed  earlier  by  frosts. 

Another  condition  which  is  equally  critical  and  as  apt  to  injure 
bearing  trees,  as  well  as  young  ones,  is  the  occurrence  of  a  fluctuating 
water-table.  The  sudden  rise  of  a  fluctuating  water-table  kills  that 
portion  of  the  root  system  which  is  located  in  the  saturated  stratum. 
In  severe  cases  where  the  major  portion  of  the  root  system  is  killed 
the  twigs  and  young  limbs  of  the  tree  later  exhibit  typical  cases  of 
"Die-back."  It  might  seem  paradoxical  that  the  top  of  the  tree 
should  dry  out  and  die  when  the  roots  stand  in  an  excessively  wet  soil, 
but  there  is  nothing  contradictory  in  the  situation  when  it  is  seen 


15 

that  the  death  of  the  major  portion  of  the  roots  makes  it  impossible 
for  the  top  to  receive  the  necessary  moisture  to  sustain  life. 

Figure  11  shows  a  tree  in  the  Elsinore  Valley  where  the  water- 
table  suddenly  rose  during  the  spring  of  1916  following  heavy  raius 
and  cloud  bursts  the  previous  winter  in  the  adjacent  mountains. 
During  the  entire  summer  of  1916  the  ground  water  in  this  grove 


-I  i 


na 


Fig.    11. — A  tree   in  the   Elsinore   Valley   showing 
sudden  rise  of  the  water-table. 


Die-back ' '    caused   by   a 


was  only  one  to  three  feet  from  the  surface,  and  at  certain  spots  in 
the  grove  springs  suddenly  appeared  from  which  water  flowed  during 
the  entire  summer.  The  water  in  the  well  on  this  grove  rose  mean- 
while, from  a  depth  of  twenty  feet  to  the  surface  of  the  ground.  The 
water  was  found  to  be  relatively  free  from  alkali,  therefore  this  injury 
may  be  ascribed  mainly  to  drowning  of  the  root  system  and  should 
not  be  confused  with  a  high  water-table  accompanied  by  alkali  salts, 
which  will  be  considered  later. 


16 


Trees  which  have  been  injured  by  an  excess  of  water  do  not  show 
the  power  of  recovery  that  the  drought-stricken  trees  show.  In  the 
former  case  the  major  portion  of  the  roots  has  been  killed,  while  in 
the  latter  case  the  roots  are  not  seriously  injured.  Although  this 
picture  was  taken  nearly  two  years  after  the  rise  in  ground  water 
and  after  the  water-table  had  fallen  to  twelve  feet  below  the  surface, 


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Fig.  12. — An  alkali-injured  tree  which  shed  the  majority  of  its  foliage  in  early 
September  and  then  leafed  out  again,  especially  at  the  terminal  buds,  and  was 
in  bloom  November  8,  1918. 


the  tree  was  not  sending  out  the  new  growth  from  the  top  or  the  root 
stock  which  would  characteristically  follow  frost  or  drought-injured 
trees.  The  year  this  picture  was  taken,  the  soil  conditions  in  this 
grove  were  favorable  to  the  growth  of  walnut  trees,  yet  the  tree  was 
barely  holding  its  own  and  in  no  sense  recovering. 

Instances  of  this  kind  are  comparatively  rare  as  the  prime  cause 
of  die-back  in  walnuts,  but  they  are  occasionally  encountered  on  lands 


17 


subject  to  overflow.  Trees  were  injured  in  the  Garden  Grove  section 
following  the  surface  flooding  of  1916  during  the  same  period  that 
the  Elsinore  trees  were  nearly  killed  by  excessive  sub-irrigation. 

4.  Alkali  injury. — Alkali  injury  is  characteristic  and  may  result 
in  the  death  of  the  walnut  tree.  On  a  soil  which  contains  alkali  in 
harmful  amounts,  the  edges  of  the  leaves  of  walnut  trees  are  often 


Fig.  13. — A  tree  in  the  same  orchard  as  the  one  shown  in  fig.  12.     Photographed 
in  June,  1919. 


burned.  This  condition  may  often  be  seen  as  early  as  June  or  July 
and  is  followed  by  the  shedding  of  the  leaves  in  August  and  early 
September.  The  sudden  and  premature  exposure  of  the  twigs  and 
limbs  to  the  hot  fall  sunlight  through  the  premature  shedding  of  the 
leaves  may  cause  severe  sunburning.  In  such  cases  also  the  terminal 
portion  of  the  twigs  put  forth  a  new  crop  of  leaves  in  October  which 
grow  until  killed  by  winter  frosts.  As  in  the  typical  frost  injury, 
the  twigs  sunburn  during  the  winter  and  much  of  the  current  year's 


18 

growth  dies  back.  Figure  12  shows  a  walnut  tree  which  shed  the 
majority  of  its  foliage  in  early  September  and  then  leafed  out  again, 
especially  at  the  terminal  buds,  and  was  in  bloom  November  8,  when 
the  picture  was  taken.  The  injury  the  following  year  to  this  late 
growth  is  shown  in  figure  13,  which  shows  the  condition  in  June,  1919, 


Fig.  14. — Two  roadside  trees,  both  of  which  are  watered  by  surplus  water  from 
adjacent  orange  groves ;  the  trees  on  the  right  receive  alkali  well  water  and  those 
on  the  left  receive  a  good  quality  river  water. 

of  a  tree  in  the  same  orchard  as  the  one  noted  above.  The  relation 
of  the  injury  to  the  salt  content  of  the  soil  may  be  seen  from  the 
following  analysis  of  soil  samples  taken  from  this  grove. 


Total 

solids 

Parts 

I>er  m 

llion 

Ft. 

SiOo 

S04 

Ca 

Na 

Mg 

K 

P04 

C03 

HC03 

Cl 

N03 

1 

1180 

91 

400 

115 

145 

30 

38 

43 

0 

183 

124 

40 

2 

1255 

109 

380 

132 

145 

35 

20 

51 

0 

122 

106.4 

7 

3 

1430 

151 

500 

240 

97 

45 

15 

14 

0 

244 

124 

6 

4 

1410 

1 30 

530 

249 

83 

40 

18 

14 

0 

320 

142 

trace 

5275 


Samples  of  soil  taken  on  the  same  day  from  an  adjoining  grove 
show  the  presence  of  only  2875  parts  per  million  of  salts,  in  contrast 
to  5275  found  in  the  grove  where  the  trees  showed  alkali  injury. 
The  difference  in  the  salt  content  of  these  soils  is  primarily  due  to 
the  quality  of  irrigation  water  used.  The  good  grove  has  been  irri- 
gated with  river  water  of  good  quality,  while  the  poor  grove  has  been 
irrigated  with  well  water  which  is  known  to  contain  relatively  large 


19 

amounts  of  soluble  material.  (A  discussion  of  the  quality  of  irriga- 
tion waters  and  of  their  effects  on  soils  will  be  found  in  a  forthcoming 
Bulletin  of  the  University  of  California,  College  of  Agriculture.) 

The  effect  of  alkali  irrigation  water  on  walnut  trees  is  seen  in 
figure  14,  which  shows  two  roadside  trees,  both  of  which  receive 
surplus  water  from  adjacent  orange  groves.  The  trees  on  the  right 
receive  alkali  (1342  parts  per  million)  well  water  and  those  on  the 
left  receive  a  good  quality  river  water. 

In  extreme  cases  of  alkali  injury,  the  whole  tree  dies  after  having 
suffered  increasing  injury  for  several  years. 


SUMMARY 

Winter  injury  or  die-back  of  walnuts  is  characterized  by  a  sudden 
death  of  the  tops  of  the  trees.  Such  injury  is  usually  first  noticeable 
during  the  early  spring  following  the  dormant  period. 

The  most  common  causes  of  the  winter  injury  or  die-back  are : 

1.  Early  autumn  frosts  kill  the  immature,  growing  shoots.  Young 
walnut  trees  are  more  subject  to  injury  from  this  cause  than  older 
trees,  because  the  former  are  usually  later  in  maturing  their  new 
wood.  Such  frosts  cause  the  foliage  to  drop  prematurely  and  injure 
the  growing  tips  of  the  twigs.  The  denuded  twigs  are  subject  to 
further  injury  from  subsequent  fall  and  winter  sunburn.  The  pres- 
ence or  extent  of  this  injury  is  usually  overlooked  until  the  following- 
spring. 

To  reduce  the  danger  from  autumn  frosts  it  is  advisable  to  withhold 
the  late  summer  irrigation  in  order  to  promote  the  early  maturity  of 
the  trees. 

2.  Winter  drought  causes  die-back  in  either  young  or  bearing  wal- 
nut groves.  Trees  suffering  from  this  condition  fail  to  make  new 
growth  in  the  spring,  except  from  the  trunks  or  main  limbs.  The 
new  growth  on  such  trees  has  lost  so  much  water  during  the  winter 
that  the  buds  are  unable  to  develop  in  spring.  The  cause  of  the  die- 
back  has  been  found  to  be  due  to  an  extremely  low  moisture  content 
of  the  soil  during  a  large  part  of  the  winter.  Under  such  conditions 
the  water  lost  from  the  young  shoots  during  the  winter  cannot  be 
replenished  by  the  root  system.  Under  such  conditions  the  shoots  die 
from  desiccation. 

Fall  and  winter  irrigation  of  the  walnut  groves  has  been  found 
to  eliminate  the  injury  from  winter  drought.  The  amount  of  irriga- 
tion will  depend  upon  the  type  of  soil,  the  amount  of  soil  moisture 


20 

present  at  the  end  of  the  harvest  season,  and  the  system  of  inter- 
cropping used. 

3.  A  high  water-table  may  be  a  contributing  factor  in  killing 
walnut  trees.  A  permanently  high  water-table  causes  the  trees  to 
prolong  their  growing  season,  with  the  result  that  they  are  killed  by 
frosts.  The  sudden  rise  of  a  fluctuating  water-table  may  kill  a  large 
part  of  the  root  system  and  produce  a  typical  die-back  in  the  tops, 
even  though  the  wood  is  mature. 

4.  Alkali  soils  containing  such  a  high  salt  content  as  to  injure 
the  root  systems  of  walnut  trees,  also  cause  the  tops  of  the  trees  to 
die  back  in  response  to  the  root  injury.  In  the  initial  stages  of  alkali 
injury  the  leaves  turn  brown  at  the  margin  and  fall  prematurely.  The 
denuded  shoots  sometimes  put  out  a  new  set  of  leaves  in  the  fall.  The 
top  of  the  trees  gradually  dies  back  until  the  entire  tree  is  lost. 

The  recovery  of  alkali-injured  trees  has  never  been  noted  by  the 
writers.  If  injury  is  being  caused  by  the  use  of  irrigation  water  con- 
taining too  much  alkali  evidently  the  use  of  such  water  should  be 
discontinued. 


